Steam generators of pressurized-water nuclear reactors comprise a casing of general cylindrical shape, within which a water box is delimited by a tube plate of great thickness, to which the bundle of tubes of the steam generator is fastened. Each of the tubes of the bundle comprises two straight parallel branches, the end parts of which are fastened inside the tube plate, usually by flanging and by welding.
When the steam generator is in operation, the tubes are in contact both with the primary fluid of the nuclear reactor, consisting of pressurized water, and with the secondary fluid consisting of feed water which evaporates. Under the operating conditions of the steam generator, the tubes are liable to suffer various kinds of damage and, in particular, corrosion by the fluids with which they are in contact. Some tubes of the steam generator can become defective during service and, for example, experience cracks causing leaks which give rise to contamination of the secondary fluid.
It is therefore necessary, during the periods of maintenance of the reactor, to take action on the steam generator and carry out the plugging of the tubes which exhibit damage liable to cause leaks.
There are known sealing plugs of the tubes of a steam generator, which comprise a body of tubular shape, closed at one of its end$ by means of a bottom, and likewise tubular clamping means introduced into the bore in the plug body in order to obtain the diametral expansion of the plug after it has been inserted into the tube of the steam generator.
The clamping means can comprise, in particular, a bush made of a metal, such as nickel, and a core made of a nickel alloy having the capacity to return to its original shape, these two components being intended to interact in order to obtain the radial expansion of the body of the plug and its fastening in the tube.
The bush and the expander core have mutually corresponding actuating surfaces which interact when the expander core is engaged into the bush in order to obtain the diametral expansion of the bush and of the plug body. The expander core is introduced into the bore in the bush when the plug is in place in the tube of the steam generator, and the actuating surfaces of the bush and of the core generally consist of frusto-conical bearing surfaces widened in the direction of the orifice in the body of the plug located opposite its bottom.
When the plug is brought to the operating temperature of the steam generator inside this generator, the expander core having the capacity of returning to its original shape experiences a diametral expansion which ensures a matching expansion of the plug body and the leak-proof closure of the tube of the steam generator.
However, such plugs which ensure effective sealing of the tubes of the steam generator have some disadvantages.
On the one hand, the nickel alloys having the capacity of returning to their original shape, which are used to make the expander core, have insufficient resistance to corrosion by the primary fluid of the reactor, at the operating temperature.
On the other hand, the leakproof clamping of the plug by pushing the expander core into the bush requires considerable exertion in order to overcome the frictional forces and deform the metal of the bush and of the plug body.
Moreover, it may prove necessary to carry out the removal of plugs which have been installed in steam-generator tubes. This is especially true when there has been preventive plugging of some tubes of the bundle which are subsequently returned to service after treatment, for example stress relief, making it possible for them to resist corrosion.
Likewise, it may prove advantageous to return to service tubes of the steam-generator bundle which have cracks, where effective processes for the repair of these tubes have recently become available.
The sealing plugs, such as those described above, can be removed by first carrying out the extraction of the core and then by drawing the plug body so as to cause contraction allowing it to be removed.
Such a removal process can be put into practice by using the device described in applicant's French Patent 87- 14326.
This two-stage process is relatively long and complicated to put into effect and requires the use of a complex tool. Moreover, there are some risks of breakage of that end part of the plug body on which a push is exerted, before the fastening and clamping part of the plug body has been separated from the inner wall of the tube of the steam generator.